Resins such as polypropylene, polyethylene, polyvinyl chloride, polystyrene, polyvinylidene chloride and the like and organic compounds such as polychlorinated biphenyl are disposed of in large amounts each year as industrial waste products or as general waste products collected from households. These resins or organic compounds, or waste plastics containing them, will hereunder be referred to by the general term xe2x80x9cwaste plasticsxe2x80x9d. Waste plastic disposal reaches an amount of about 4 million tons per year as industrial waste products and about 4 million tons per year as general waste products.
The present invention relates to a recycling treatment process for waste plastics.
The majority of conventional processes for recycling treatment of waste plastics involve combustion incineration and land-fill disposal. Combustion disposal results in damage to incinerators due to the large heat release, while chlorine-containing waste plastics present the problem of dealing with chlorine in the exhaust gas. waste plastics are not decomposed by microbes or bacteria in the soil, and this has led to a lack of land-fill sites and an increasing environmental burden. In consideration of the environment, it has been attempted to employ recycling techniques involving no incineration or land-fill disposal. Current recycling methods that avoid incineration include reuse as plastic raw materials and reuse of gas components obtained by thermal decomposition as fuel or chemical raw materials.
Among such methods of reuse of gas components obtained by thermal decomposition as fuel or chemical raw materials there is already known a method that utilizes waste plastics as reducing agents in blast furnaces and as part of a steel production process (Japanese Examined Patent Publication SHO No. 51-33493) and, recently, a variety of means have been developed for more efficiently realizing this purpose. (See, for example, Japanese Unexamined Patent Publication HEI No. 9-170009, No. 9-137926, No. 9-178130, No. 9-202907, Japanese Patent No. 2765535.)
For disposal of waste plastics in blast furnaces, it is necessary to consider the reduced blast furnace productivity due to blowing in of large amounts of waste plastics, and the chlorine that is inevitably included in the waste plastics.
Specifically, when waste plastics are loaded into a blast furnace at over 10 kg per ton of hot metal produced, there is a notable loss in productivity of the pig-iron as the blast furnace core tends to be inactivated. A conventional measure has been to set a limit of 10 kg per ton of hot metal in blast furnaces. Polyvinyl chloride, polyvinylidene chloride and polychlorinated biphenyl all contain chlorine, and on average chlorine is included in industrial waste plastics at about a few dozen wt % and in general waste plastics at about a few dozen wt %; both industrial waste plastics and general waste plastics still include chlorine at an average of a few wt % even after having undergone separation treatment. When such waste plastics are loaded directly into a blast furnace, the chlorine components of the waste plastics are converted to chlorine-based gases such as chlorine and hydrogen chloride by thermal decomposition, thus causing such problems as corrosion of the blast furnace body shell, corrosion of the stave cooler for cooling of the blast furnace, corrosion of the top exhaust gas apparatus of the blast furnace and corrosion of the generating equipment of the blast furnace. Consequently, chlorine-containing waste plastics have been removed beforehand or the chlorine components removed from the waste plastics before loading into blast furnaces.
A method of thermal decomposition treatment of waste plastics in coke ovens has long been known as part of steel production processes (Japanese Examined Patent Publication SHO No. 49-10321, Japanese Unexamined Patent Publication SHO No. 59-120682), and recently a variety of means have been developed for more efficiently accomplishing treatment of waste plastics, such as loading methods that take into consideration coke strength. (See, for example, Japanese Unexamined Patent Publication HEI No. 8-157834). Coke ovens are facilities for dry distillation of coal, and they also serve as recycling plants that can dry distill waste plastics to obtain fuel gas, tar and coke.
As with treatment in blast furnaces, when waste plastics are processed in coke ovens it has also been necessary to consider the reduced coke oven productivity due to the waste plastics, and the chlorine that is inevitably included in the waste plastics.
When loaded into a coke oven in admixture with coal, under current circumstances the coke quality is drastically reduced with loading of over 10 kg per ton of coal. Consequently, while it should be theoretically possible to process 10 kg per ton of coal in a coke oven, when waste plastics that inevitably include chlorine at about 3-5 wt % are directly loaded into a coke oven the chlorine may remain in the coke, leading not only to concerns of corrosion of exhaust gas lines by chlorine-based gases produced by thermal decomposition but also to concerns of inclusion of chlorine-based gas in the tar and coke oven gas by-products; for loading into coke ovens, therefore, it is usually necessary to first remove the chlorine components by thermal decomposition before loading, as described in Japanese Unexamined Patent Publication HEI No. 7-216361, or to first remove the chlorine-based resins by gravity separation or the like before loading of waste plastics into the coke oven, as described in Japanese Unexamined Patent Publication HEI No. 8-259955, for which reasons treatment in coke ovens has not been attempted in practice.
In methods where waste plastics are used as reducing agents in blast furnaces as part of the steel production process, there is a limit to the amount of waste plastics that can be blown into a blast furnace through the tuyere, and in consideration of reduced blast furnace productivity, since the mean productivity is about 2 t/d/m3 even in a large-sized blast furnace with a volume of 4000 m3, it is not possible to process more than 30,000 tons of waste plastics per year even with maximum blowing into the blast furnace, and this makes it impossible to meet social demands for recycling of the large volumes of waste plastics that are disposed of each year. Furthermore, prior removal of chlorine-containing waste plastics or removal of the chlorine components in the waste plastics has complicated the procedure, leading to increased treatment costs. Waste plastics collected in cities usually contain about 3-5 wt % of chlorine components after pre-treatment by magnetic separation, aluminum separation, and the like. This is largely due to the 6-10 wt % of polyvinyl chloride contained in the waste plastics. Corrosion of blast furnaces by chlorine-based gas will generally occur unless the chlorine content is reduced to under 0.5 wt %. Methods are therefore employed to remove the chlorine components beforehand as chlorine-based gas by heating at about 300xc2x0 C., or to separate the lightweight plastics from the heavyweight plastics by gravity separation using a centrifugal separator, for example, and loading only the lightweight plastics with low chlorine contents into the blast furnace. However, this has not been employed to any great extent because of the extremely high cost of dechlorinating all collected waste plastics with dechlorination apparatuses and the technical difficulty of dechlorination from 3-5 wt % to 0.5 wt %. Instead, it has become common to employ the method of separating lightweight plastics from heavyweight plastics by gravity separation using a centrifugal separator, for example, and loading only the lightweight plastics with low chlorine contents into the blast furnace. Nevertheless, this method has also presented some problems. These will be explained using a centrifugal separator as an example. It is impossible to achieve ideal separation of 100 kg of foreign matter-removed waste plastics (containing 10 kg of vinyl chloride and 5 kg of chlorine by weight) with a centrifugal separator, i.e., to obtain 90 kg of matter with a 0% chlorine content as the lightweight portion and 10 kg of matter with a 50% chlorine content as the heavyweight portion (the chlorine content of polyvinyl chloride is approximately 57 wt %). At the current time, it is only possible to obtain 50 kg of matter with a chlorine content of 0.5% as the lightweight portion and 50 kg of matter with a chlorine content of 9.5% as the heavyweight portion or, even under the most ideal conditions, to separate 70 kg of matter with a chlorine content of 0.5% as the lightweight portion and 30 kg of matter with a chlorine content of 15.5% as the heavyweight portion. In this case, the waste plastics containing chlorine at 9.5-15.5 wt % cannot be further dechlorinated to a chlorine content of 0.5 wt % or lower, and the separated heavyweight waste plastics (corresponding to 30-50% of the waste plastics before separation) are treated as residue and have had to be disposed of by land-fill, for example. Treatment as residue not only incurs treatment costs of about 30,000 yen per ton of residue but also highlights the low recycling efficiency of waste plastics, such that this recycling method fails to meet the social demands for recycling.
As mentioned above, the method of utilizing waste plastics in coke ovens as a part of the steel production process is recognized as requiring removal of chlorine components by thermal decomposition prior to loading or removal of chlorine-based resins by gravity separation or the like before loading of waste plastics into the coke oven, and therefore treatment in coke ovens has not been carried out in a practical manner. As explained above, prior removal of chlorine components is a problem for waste plastic recycling in blast furnaces because of the high cost of dechlorination, the high cost of heavyweight waste plastic treatment and the low recycling efficiency, which are problems that have prevented application of this method.
Even with removal of the chlorine components before loading and loading of the maximum possible amount of waste plastics in a coke oven with 100 chambers, it is possible to process no more than 10,000 tons per year, which is a volume that cannot meet the social demands of recycling of waste plastics that are disposed of in large amounts each year.
The present invention provides a means of solving these problems of the prior art and, specifically, it not only provides a method of recycling waste plastics as gas, tar and coke by increasing the waste plastic treatment capacity of conventional blast furnaces in terms of treatment capacity in the ironmaking step of steel production without investment of expensive equipment for the ironmaking step, but also provides a treatment process that can achieve greater recycling efficiency for waste plastics by reducing or eliminating the burden of the prior dechlorination step which has been indispensable according to the prior art. The gist of this method is based on a process that involves loading waste plastics into a thermal decomposition furnace without prior removal of the chlorine components and contacting the thermally decomposed gas containing chlorine-based gas with ammonia water, in combination with waste plastic treatment in a blast furnace. A coke oven may be conveniently employed as the thermal decomposition furnace since the ammonia water produced from coal can be utilized. However, the present invention is not limited to coke ovens and can be carried out with any thermal decomposition furnace, ammonia source and blast furnace. The present inventors have discovered the following three embodiments in which a coke oven is used as the thermal decomposition furnace.
The first embodiment is a method whereby waste plastics containing chlorine components are separated into high level and low level component portions by gravity separation, for example, and the high level of chlorine component portion is loaded into a coke oven for thermal decomposition treatment while the low level of chlorine component waste plastics are loaded into a blast furnace for thermal decomposition treatment (FIG. 3A).
The second embodiment is a method whereby a portion of waste plastics is subjected to prior dechlorination treatment before loading into a blast furnace and a portion or all of the remainder is loaded into a coke oven without prior dechlorination treatment (FIG. 3B).
The third embodiment is a method whereby waste plastics, without prior removal of the chlorine components, are loaded into a blast furnace in an acceptable chlorine concentration range for treatment, and a portion or all of the remainder is loaded into a coke oven for treatment, also without prior removal of the chlorine components (FIG. 3C).